Exhaust system

ABSTRACT

A simplified very low height sound attenuating muffler for flowing gases is formed from an outer tube shaped into an oval cross section to engage and be welded to a perforated straightthrough gas flow tube and then the ends of the outer tube and selected intermediate portions are compressed into contact with each other to form closures at opposite ends of the muffler and at interior points which divide the muffler into chambers of various volumes that may be keyed to the frequencies to be attenuated.

United States Patent Heath et al.

[ Mar. 14, 1972 [54] EXHAUST SYSTEM 2,184,891 12/1939 Boume 181/59 721 Inventors: Robert A. Heath; Ronald J. Manoia, both Z 2 $9 5:322 13cm", Mlch- 3,289,785 12/1966 Walker 1 81/48 73 Ass ne Walker Manulacturin C R l 1 lg e Wis g ompany' acme Primary Examiner-Stephen J. Tomsky AttorneyHamess, Dickey & Pierce [22] Filed: Oct. 13, 1969 [21] Appl. No.: 868,283 [57] ABSTRACT Relat d -S- Applica ion Dali! A simplified very low height sound attenuating muffler for flowing gases is formed from an outer tube shaped into an oval [63] Commuanon of 531245 1966' cross section to engage and be welded to a perforated straightthrough gas flow tube and then the ends of the outer tube and [52] US. CL... ..l8l/48, 181/61 Selected intermediate portions are compressed into Contact [51] [1 11. CI ..F0ln U02, F0111 7/18 i h each other to form closures at opposite ends of the [58] Field of Search ..181/48, 47, 54, 59, 61, 63 fl and at interior points whiCh divide the uffl i chambers of various volumes that may be keyed to the frequencies [56] References Cited to be attenuated.

UNITED STATES PATENTS 4 Claims, 17 Drawing Figures 2,075,265 3/1937 Bourne ..l81/59 /J a; a /4 7 /4/ 4 /74 /z: //J (fl) H 77 i /7 5 u l. 74 M4 42/ 42! I PATENTEDMAR 14 I972 SHEET 3 OF 5 Faen PAIENTEIJMAR 14 :912 3,648 803 sum 5 [IF 5 INVENTORS.

EXHAUST SYSTEM RELATED APPLICATION This application is a continuation of Ser. No. 531,245, filed Mar. 2, 1966.

BRIEF SUMMARY OF THE INVENTION Since modern motor vehicle design requires a high degree of engine exhaust system silencing in a minimum of space, it is a principal object of this invention to provide such a compact but highly effective exhaust silencing system.

It is a further object of the invention to provide an improved silencing device having a relatively low height and yet capable of silencing a wide range of frequencies in a minimum overall volume.

It is a still further object of this invention to provide a silencing device having a relatively small number of components and yet capable of silencing a large number or range of frequencies.

It is another object of this invention to provide an improved method for forming a silencing device that is capable of attenuating a wide range of silent frequencies.

It is yet another object of this invention to provide an improved method that lends itself to the low cost formation of a silencing device for an internal combustion engine.

An exhaust system for an internal combustion engine includes at least one silencing device embodying the invention disclosed herein. The inventive silencing device is comprised of an outer shell formed from an integral, elongated tubular member defonned into a generally oval cross-sectional contour. An inner tube extends the length of the outer shell and forms an exhaust inlet in one end of the outer shell and an exhaust outlet in the other end of the outer shell. Means form closures for opposite ends of the outer shell around the exhaust gas inlet and exhaust gas outlet. The outer shell is in engagement with the opposite sides of the inner tube to define at least one longitudinally extending chamber therebetween. The plurality of perforate openings in the inner tube provide for gas communication with the chamber for silencing.

The method of forming an exhaust silencer of the type described in the immediately preceding paragraph and embodying this invention comprises the steps of deforming a relatively large diameter tubular member into a substantially oval shape. A smaller diameter tubular member is inserted into the larger diameter tubular member with opposite sides in engagement with the deformed larger member to define at least one silencing chamber between the members. End closures are formed for the larger member around the smaller member to enclose a chamber between the members. The opposite ends of the smaller member forming exhaust gas inlet and outlet bushings in respective ends of the silencer.

Other objects and advantages of this invention will become more apparent as this description proceeds, particularly when considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of an automotive exhaust system embodying this invention;

FIG. 2 is an enlarged cross-sectional view of one of the silencing devices of the system in FIG. 1 taken substantially in the plane of FIG. 1;

FIG. 3 is an enlarged cross-sectional view taken along the line 3-3 of F IG. 2;

FIG. 4 is an enlarged cross-sectional view, taken in the plane of FIG. 1, showing another of the silencing devices;

FIG. 5 is an enlarged cross-sectional view taken along the line 5-5 of FIG. 4;

FIG. 6 is an enlarged cross-sectional view, taken in the plane of FIG. 1, showing the remaining silencing device;

FIG. 7 is a cross-sectional view taken generally along the line 7-7 of FIG. 6;

FIG. 8 is a perspective view, with portions broken away, of a part of the silencing device shown in FIGS. 2 and 3;

FIG. 9 is a cross-sectional view, in part similar to FIGS. 2 and 4 and showing another embodiment of the invention;

FIG. 10 is a cross-sectional view taken along the line 10-10 DETAILED DESCRIPTION OF THE INVENTION Referring now in detail to FIG. 1, a portion of an internal combustion engine is identified generally by the reference numeral 21. The engine has an exhaust manifold 22 from which exhaust gases are delivered to an exhaust system embodying this invention and identified generally by the reference numeral 23. If the engine 21 is of the V-type, a similar exhaust system may be provided for each bank of cylinders or only one system may serve both banks. The system 23 also may be used with an in-line engine.

The exhaust system 23 is comprised of a first silencing device 24 having an inlet bushing 25 that is connected by means of an exhaust inlet pipe 26 to the engine exhaust manifold 22. An outlet bushing 27 of the first silencing device 24 is connected to an inlet bushing 28 of a second silencing device 29 by means of a short length of pipe 31. The second silencing device 29 has an outlet bushing 32 that is connected to an inlet bushing 33 of a third silencing device 34 by means of a pipe section 35. An outlet bushing 36 of the third silencing device 34 is connected to a short tailpipe 37 for discharge of the silenced exhaust gases to the atmosphere.

Referring now specifically to FIGS. 2, 3, and 8, the first silencing device 24 is comprised of an outer shell 38 which, as will be discussed below, is comprised of a relatively large diameter integral tubular member that is deformed into a generally oval cross-sectional shape. Contained within the outer shell 38 is an inner tube 39 that is engaged on opposite sides thereof by the deformed portion of the outer shell 38 (FIG. 3). Since the remaining portion of the outer shell 38 is not in engagement with the inner tube 39, a pair of chambers are formed on opposite sides of the silencing device 24.

One side of the outer shell 38 is formed with integral pinched-in sections 41, 42, 43, 44, 45, 46 and 47 formed by permanently deforming the portion of the outer shell 38 on the respective side of the silencing device 24 so that the outer shell sides sealingly engage the inner tube 39 around approximately of its circumference. In addition, the facing surfaces of the outer shell 38 at the pinched-in sections 41 through 47 engage each other to form, in effect, partitions that divide the chamber at one side of the device 24 into a plurality of longitudinally spaced silencing chambers 51, 52, 53, 54, 55, 56, 57 and 58. The chambers 51 and 58 at opposite ends of the silencing device 24 are closed by pinched-in sections 59 and 61 of the outer shell 38 which form closures for the opposite ends of this side of the silencing device 24 in the same manner as the pinched-in sections 41 through 47 form partitions.

The side opposite the chambers 51 through 58 of the outer shell 38 is formed with pinched-in sections 62, 63 and 64 that define longitudinally spaced silencing chambers 65, 66, 67 and 68. The pinched-in sections 63 and 64 are similar in configuration to the sections 41 through 47 in that they engage the inner tube 39 around one-half of its circumference and form partitions between the chambers 66, 67 and 68.

An intermediate tube 69 extends parallel to the inner tube 39 through the pinched-in section 62 and is affixed, as by spot welds indicated by the Xs, to the inner tube 39. The opposite ends of the intermediate tube 69 open into the chambers 65 and 66. Opposing faces 71 of the outer shell 38 pinched-in section 62 engage the intermediate pipe 69 and adjacent side of the inner tube 39 so as to form a complete partition between the chambers 65 and 66. The intermediate tube 69, however, extends through the partition thus formed and provides for fluid communication between the chambers 65 and 66. The outer end of the chamber 65 is closed by means of a pinched-in section 72 similar to the sections 59 and 61 at opposite side of the device. In a like manner, the rear end of the chamber 68 is closed by means of a pinched-in section 73.

At the inlet end of the first silencing device 24, the exhaust gas inlet bushing 25 is formed by a pipe having a reduced diameter section 74 that terminates at an outstanding flange 75. The pinched-in sections 59 and 72 of the outer shell 38 engage the reduced diameter section 74 completely around its circumference so as to form effective closure for this end of the outer shell 38. The inner tube 39 is slidingly received in the reduced diameter section 74 and may be affixed to it, as by the spot welds noted by the xs in the drawing. In a like manner, the outlet bushing 27 at the opposite end of the first silencing device 24 is formed by a pipe having an enlarged diameter portion 76 that is sealingly engaged around its periphery by the pinched-in sections 61 and 73. The opposite end of the inner tube 39 is received within the outlet bushing portion 76 and may be affixed to it, as by spot welds indicated by the reference numeral X. If desired, the opposite sides of all or any of the pinched-in sections may be further spot welded to each other.

The inner pipe 39 is provided with a first series of louvered openings 77 that extend completely around its circumference and substantially the full length of the chambers 51 and 65. The first series of louvered openings provides for communication of the exhaust gases flowing through the inner tube 39 I into the chambers 51 and 65. A second series of louvered openings 78 is formed around approximately 180 of the inner tube 39 in the area between the pinched-in sections 41 and 46 of the outer shell 39. The louvered openings 78 thus provide for communication of the exhaust gases with the chambers 52 through 56. A third series of louvered openings 79 are formed around the circumference of the inner tube 39 between the pinched-in sections 46 and 63 and the pinched-in sections 61 and 73. The louvered openings 79 thus provide for exhaust gas communication with the chambers 57, 58, 67 and 68.

The louvered openings into the chambers 51 through 58, 67 and 68 cause these chambers to function as high frequency sound attenuating chambers known in the art as spit chambers. These chambers will attenuate a relatively broad band of high frequency noises, the band determined by the volume of individual chambers and the size and spacing of the louvered openings into them. By altering the volumes of the chambers 51 through 58, 67 and 68 as well as the spacing and size of the louvers 77, 78 and 79, a broad band of high frequency ranges may be attenuated within the silencing device 24.

The chamber 65 will also function somewhat as a spit chamber due to the number of louvered openings into it. The exhaust pressure pulses in the chamber 65 will be transmitted through the tuning neck formed by the intermediate tube 69 into the chamber 66 which is otherwise closed. Thus, the chamber 66 and the tuning neck 69 act as a Helmholtz tuner for attenuating a desired frequency. The louvered openings 77 broad band this tuner to some extent. Preferably, the louvered opening 77 in the chamber 65 is located at an anti-node of the frequency silenced by the Helmholtz tuner.

Referring now in detail to FIGS. 4 and 5, the silencing device 29 is similar in some respects to the silencing device 24 and includes an outer shell 81 that is formed from a large diameter piece of tubing that is pennanently deformed into a generally oval shape and through which an inner tube 82 extends. The inner tube 82 is engaged on diametrically opposite sides by the outer shell 81 so as to form two separate chambers on opposite sides of the device 29. The chambers are divided into a plurality of longitudinally spaced chambers by pairs of pinched-in sections 83, 84, 85 and 86. As in the silencing device 24, the pinched-down sections 83-87 form partitions that isolate sound attenuating chambers 92 through 99, 101 through 104 from each other. As in the preceding embodi ment, the opposite ends of the pipe 81 are telescopically received within the inlet bushing 28 and the outlet bushing 32.

End closures for the respective ends of the silencing device 29 are formed by pinched-in sections, indicated generally by the reference numeral 105.

A first series of louvered openings 106 are found around the circumference of the inner pipe 81 for substantially the full length of the chambers 92 through 96 and 98, 99, 100, 102 and 103. Thus, these chambers function as spit chambers for attenuating high frequency sounds. These chambers may have the same or different volumes but in the depicted embodiment each of the respective pairs have the same volume. The inner pipe 82 has a second bank of louvered openings 107 formed around its circumference and opening into the chambers 97 and 104. The louvered openings 107 do not extend the full length of the chambers 97 and 104 and these chambers function as quarter wave length tuners for silencing a predetermined frequency and all odd harmonics of this predetermined frequency. The frequency tuned is equivalent to the frequency that has a wave length equal to one-quarter of the length of the tuning chamber, in this case the distance from the center of the louvered openings 107 to the closed end formed by the pinched-in sections 105. Although the chambers 97 and 104 have the same lengths, it is to be understood that different lengths may be employed by changing the location of either of the pairs of pinched-in sections 87 or 105.

Referring now to FIGS. 6 and 7, the silencing device 34 is designed to attenuate a broad band of low and intermediate frequencies and also includes a spit chamber device for attenuating high frequency sounds. The device 34 has an inner tube 111 that is slidably received at its opposite ends in the inlet and outlet bushings 33 and 36 and may be affixed, as by spot welding, to one or both of them. An outer shell 112 of generally circular configuration is necked down at each end and affixed, as by a circumferential weld, to the bushings 33 and 36. A volume 113 or chamber is formed between the outer shell 112 and inner tube 111 which functions as sound attenuating expansion chamber. A plurality of louvered openings 114 are formed along a major portion of the length of the inner tube 111 around the circumference. These openings allow exhaust gas to flow into the chamber 113 for attenuating a broad band of low and intermediate frequencies.

A shell 1 15. also encircles the rearwardmost end of the inner tube 111 to define a second tuning volume 116 within the device 112. A bank of louvers 117 is formed in the portion of the inner tube 114 within the shell so that the chamber 1 16 acts as a high frequency attenuating spit chamber.

It has been found that the described system will effectively attenuate substantially all objectionable noises from a certain design of automotive engine. The silencing devices 24 and 29 are particularly effective in this regard inasmuch as they provide a plurality of different tuning devices within a relatively small overall height. That is, they include various size spit chambers, quarter wave length tuners and Helmholtz tuners. It is to be understood that various modifications may be made in the depicted devices, for example, changing the volume of the individual chambers by altering the location of the pinched-in sections which define the partitions between the chambers. In the devices 24 and 29 the inner tube was centrally disposed within the outer shell so as to define two rows of chambers on either side of the device. If desired, the inner tube could be offset to one side of the outer shell so as to provide different volume for the chambers at each side or to provide a single row of chambers having larger volumes if even more offset is desired. Such a device is shown in FIGS. 9 and 10 and is identified generally by the reference numeral 121.

The silencing device 121 is comprised of an outer shell 122 which, as in the devices 24 and 29, is formed from a large diameter piece of integral tubing that is deformed into a generally oval cross-sectional shape. An inner tube 123 is disposed at one side of the outer shell 122 and has an outer diameter that substantially conforms to the arcuate side of the outer shell 122. The inner tube extends beyond one end of the outer shell 122 and is formed with an outwardly flared portion 124 that forms an inlet bushing. The opposite end of the inner tube 123 extends beyond the opposite end of the outer shell 122 and forms a reduced diameter outlet bushing 125.

An elongated volume is formed at the one side of the outer shell 122 adjacent the inner tube 123 and the ends of this volume are closed by pinched-in sections 126 and 127 that form closures for the respective ends of the outer shell 122 as in the previously described embodiments. In addition, a plurality of pinched-in sections 128, 129 and 130 are formed at longitudinally spaced positions to divide the chamber into a plurality of silencing chambers 131, 132, 133 and 134. Onehalf of the circumference of the inner tube 123 may be provided with a bank of louvers 135 that extends substantially the full length of the outer shell 122. The chambers 131 through 134 will function as broad banded silencing devices for intermediate and high frequency sounds. As in the previously described embodiments, the volumes of these chambers may be altered and the size and number of the louvered openings into each chamber may be selected so as to accomplish the desired degree of tuning.

A still further embodiment of the invention is shown in FIG. 11 wherein a silencing device is identified generally by the reference numeral 141. As in the previously described embodiments, the silencing device 141 comprises a generally oval elongated outer shell 142 in which an inner tube 143 is positioned. The inner tube 143 is centrally disposed so as to provide two longitudinally extending chambers on opposite sides thereof that are separated by the contact between the outer shell 142 and the diametrically opposite sides of the inner tube 143. Exhaust gas inlet and outlet bushings 144 and 145 are formed in any suitable manner at the opposite ends of the inner tube 143 externally of the outer shell 142. The opposite ends of the outer shell 142 are closed, as by pinched-in sections 146 of the same type described in the preceding embodiments.

In this embodiment, the volume at one side of the device 141 is divided into two longitudinally spaced chambers 147 and 148 by means of a pinched-in section 149. The chamber at the opposite side of the device 141 is divided into two chambers 151 and 152 by means of a pinched-in section 153 of the outer shell 142. Although in this embodiment the chambers 147 and 151 and the chambers 148 and 152 have the same volumes, different volumes could be provided by ap-' propriately positioning the pinched-in sections 149 and 153. In addition, a greater number of chambers could be provided if desired.

At one end of the outer shell 142, the inner tube 143 is provided with a relatively short bank of louvered openings 154 that open into the chamber 151. The openings 154 extend around one-half or less of the circumference of the inner tube 143 and open only into the chamber 151. Thus, the chamber 151 functions as a quarter wave length tuner for silencing the frequency having a wave length approximately equal to four times the distance from the center of the bank of louvers 154 to the pinched-in section 153. On its opposite side, the inner tube 143 has a short bank of louvers 155 that open only into the chamber 147. The bank of louvers 155 is positioned between the ends of the chamber 147 so that the chamber 147 in effect acts as two quarter wave length tuners within a single chamber. These tuners silence the frequencies having wave lengths equal to four times the distance from the center of the bank of louvers 155 to each of the pinched-in sections 146 and 149.

A relatively short bank of louvers 156 is formed in the portion of the inner tube 143 opening into the chamber 152 and adjacent the pinched-in section 153. Thus, the section 152 acts as a relatively high frequency quarter wave length tuner. A somewhat longer bank of louvers 157 is formed on the opposite side of the inner tube 143 opening into the chamber 148. This device will attenuate a somewhat broader band of frequencies in the chamber 152.

As in the previously described embodiments, the device 141 shown in FIG. 11 is susceptible of incorporating certain other features and the frequencies tuned may be changed by altering the volumes of the respective chambers or the locations of the louvers that open into each of the chambers and the number of louvers. Other forms of fiow passages than louvered openings may be employed, but the louvered openings have the effect of broadening the range of frequencies silenced by a specific tuner.

As has been noted in the embodiment shown in FIG. 11, the chambers 147 and 151 and the chambers 148 and 152 have the same volume. Different volumes may be employed by changing the locations of the pinched-in sections, as shown in FIG. 12. In this embodiment, a silencing device, indicated generally by the reference numeral 161, is comprised of an outer shell 162 and an inner tube 163 with pinched-in sections 164, 165, 166, 167, 168 and 169 forming end closures and dividing the space between the outer shell 162 and inner tube 163 into four discrete silencing chambers 171, 172, 173 and 174. The chambers 171 through 174 each have different volumes. As in the embodiment shown in FIG. 11, banks of louvers 175, 176, 177 and 178 open into the chambers 171 through 174, respectively. The chambers 172 through 174 each function as quarter wave length tuners and the chamber 171 function as two quarter wave length chambers due to the eccentric location of the louvers 175.

The method of forming the silencing devices identified generally by the reference numerals 24, 29, 121, 141 or 161, will now be described by particular reference to FIGS. 13 through 17. As has been previously noted, the outer shells of these devices are formed by taking a large diameter tubular member, indicated generally by the reference numeral 191 in FIG. 13, and permanently deforming it into an elongated oval shape,indicated generally by the reference numeral 192 (FIG. 14). Any suitable bending machine may be used for this purpose. A cylindrical inner tube 193 having an outer diameter substantially equal to the height of the deformed section 192 is then inserted into the deformed outer shell 192. Depending upon whether one or two longitudinal rows of silencing chambers are provided, the tube 193 is centrally disposed within the deformed outer shell 192, offset slightly to one side thereof or positioned at one side thereof. The tube 193 is formed with the louvers that will open into the respective silencing chambers before insertion into the outer shell 192, although in some instances the louvers may be formed after assembly.

The outer shell 192 is then permanently deformed to provide pinched-in sections 194, 195, 196, 197, 198 and 199. The pinched-in sections may be formed simultaneously in pairs or singly by means of a suitable bending press. If the sections are aligned, as the sections 194 and 195, it is most convenient to form them simultaneously (FIG. 16). At the same time the pinched-in sections 194 through 199 are formed, the opposing surfaces of the outer shell 192 may be spot welded together. The spot welding may be done later or may be omitted altogether.

The sections 196, 197, 198 and 199 form the end closures for the muffler and the sections 196 and 197 form the intermediate partitions.

While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification.

We claim:

1. A silencing device comprised of an outer shell formed from an elongate integral tubular member of a generally oval cross-sectional contour, a straight through inner gas flow tube extending the length of said outer shell and forming an exhaust gas inlet at one end of said outer shell and an exhaust gas outlet at the other end of said outer shell, opposite sides of the shell being in engagement to form end closures at opposite ends of said outer shell around said exhaust gas inlet and said exhaust gas outlet, said outer shell being in engagement with opposite sides of said inner tube along the full length thereof to define a longitudinally extending space between each side of the tube and each side of the shell, each of said spaces being subdivided into a plurality of longitudinally spaced silencing chambers by means of longitudinally spaced integral sections of the outer shell that are in engagement with the respective side of the inner tube and with each other to define partitions separating said silencing chambers, said inner tube having spaced patches of perforations formed therein so that substantially all of the silencing chambers have at least one patch opening into it, at least one perforation patch being spaced substantially from the opposite ends of the silencing chamber into which it opens, said patch being confined to a relatively short length of said inner tube for forming two effective silencing subchambers within said silencing chamber located on opposite sides of said patch, each of said effective subchambers having a length substantially equal to the distance between said patch and the respective opposite end of said subchamber.

2. A silencing device comprised of an outer shell formed from an elongate integral tubular member of a generally oval cross-sectional contour, a straight through inner gas flow tube extending the length of said outer shell and forming an exhaust gas inlet at one end of said outer shell and an exhaust gas outlet at the other end of said outer shell, opposite sides of the shell being in engagement to form end closures at opposite ends of said outer shell around said exhaust gas inlet and said exhaust gas outlet, said outer shell being in engagement with opposite sides of said inner tube along the full length thereof to define a longitudinally extending space between each side of the tube and each side of the shell, each of said spaces being subdivided into a plurality of longitudinally spaced silencing chambers by means of longitudinally spaced integral sections of the outer shell that are in engagement with the respective side of the inner tube and with each other to define partitions separating said silencing chambers, said inner tube having spaced patches of perforations formed therein so that substantially all of the silencing chambers have at least one patch opening into it, and four chambers having said perforate patches therein which are materially shorter in length than the chambers whereby the chambers can be tuned to attenuate a desired frequency.

3. A device as set forth in claim 2 wherein said four chambers are of four different lengths.

4. A device as set forth in claim 3 wherein the perforate patch in at least one of the four chambers is located at an intermediate point in the chamber which is spaced unequal distances from opposite ends of the chamber. 

1. A silencing device comprised of an outer shell formed from an elongate integral tubular member of a generally oval crosssectional contour, a straight through inner gas flow tube extending the length of said outer shell and forming an exhaust gas inlet at one end of said outer shell and an exhaust gas outlet at the other end of said outer shell, opposite sides of the shell being in engagement to form end closures at opposite ends of said outer shell around said exhaust gas inlet and said exhaust gas outlet, said outer shell being in engagement with opposite sides of said inner tube along the full length thereof to define a longitudinally extending space between each side of the tube and each side of the shell, each of said spaces being subdivided into a plurality of longitudinally spaced silencing chambers by means of longitudinally spaced integral sections of the outer shell that are in engagement with the respective side of the inner tube and with each other to define partitions separating said silencing chambers, said inner tube having spaced patches of perforations formed therein so that substantially all of the silencing chambers have at least one patch opening into it, at least one perforation patch being spaced substantially from the opposite ends of the silencing chamber into which it opens, said patch being confined to a relatively short length of said inner tube for forming two effective silencing subchambers within said silencing chamber located on opposite sides of said patch, each of said effective subchambers having a length substantially equal to the distance between said patch and the respective opposite end of said subchamber.
 2. A silencing device comprised of an outer shell formed from an elongate integral tubular member of a generally oval cross-sectional contour, a straight through inner gas flow tube extending the length of said outer shell and forming an exhaust gas inlet at one end of said outer shell and an exhaust gas outlet at the other end of said outer shell, opposite sides of the shell being in engagement to form end closures at opposite ends of said outer shell around said exhaust gas inlet and said exhaust gas outlet, said outer shell being in engagement with opposite sides of said inner tube along the full length thereof to define a longitudinally extending space between each side of the tube and each side of the shell, each of said spaces being subdivided into a plurality of longitudinally spaced silencing chambers by means of longitudinally spaced integral secTions of the outer shell that are in engagement with the respective side of the inner tube and with each other to define partitions separating said silencing chambers, said inner tube having spaced patches of perforations formed therein so that substantially all of the silencing chambers have at least one patch opening into it, and four chambers having said perforate patches therein which are materially shorter in length than the chambers whereby the chambers can be tuned to attenuate a desired frequency.
 3. A device as set forth in claim 2 wherein said four chambers are of four different lengths.
 4. A device as set forth in claim 3 wherein the perforate patch in at least one of the four chambers is located at an intermediate point in the chamber which is spaced unequal distances from opposite ends of the chamber. 